Autoclave circuits are used, for example, to recover gold from refractory sulphidic ores and base metals such as nickel, copper and cobalt from oxides or sulphide mineral ores. An aqueous mixture of water, dissolved minerals and solid particulate (slurry) typically leaves an autoclave under high temperature and pressure and is passed to one or more flash vessels in series or parallel. In the flash vessel(s), the pressure of the slurry is reduced and the aqueous fraction rapidly boils to achieve thermodynamic equilibrium, thereby cooling the slurry. Eventually, the slurry pressure and temperature are reduced, by one or more stages of flashing, to atmospheric pressure for further processing.
Once the slurry has reached the flash vessel, the pressure reduction is accomplished by one or more choke valves and “flash tubes” through which the slurry enters the flash vessel. The flash tube is a diffuser nozzle with a conical, frustum-shaped passageway. The flash tube is generally mounted vertically downward inside the flash vessel. While the slurry is in the liquid state before entering the flash tube, the pressure reduction that occurs as the slurry enters the flash tube results in the formation of a gas/vapor phase such that the slurry jet appears to enter the flash vessel as a mixture of liquid-phase slurry, a gas/vapor phase, dissolved minerals and solid particles. Even though a mixture of liquid, gas/vapor and solid enters the flash vessel, the three phases may depart the flash vessel separately.
In a given flash vessel, the gas/vapor that evolves from flashing the aqueous fraction exit from one or more gas/vapor outlets usually located at the top of the vessel. Upon exit from the given flash vessel, the gas/vapor may be recovered and recycled. Recycling of steam may, for instance, involve condensing the steam with autoclave feed slurry in a complementary direct contact condenser for preheating the autoclave ore feed slurry. An inventory of liquid-phase slurry remains in the flash vessel in a slurry pool that can act to dissipate the momentum of the jet from the flash tube. Excess liquid-phase slurry exits the flash vessel via one or more slurry outlets.
A properly designed flash tube should produce a flow field having an exit pressure equal to the vapor pressure inside the flash vessel. If the flash tube exit pressure is greater than the vapor pressure in the flash vessel (back pressure), the exiting mixture of liquid-phase slurry and gas/vapor suddenly expands, creating a flared jet that impinges on the walls of the flash vessel. Normally, the vessel walls are lined with an application-specific lining. The slurry jet resulting from back pressure may lead to erosion of the lining. In the other case, when the flash tube exit has a pressure lower than the vapor pressure in the flash vessel, the exiting mixture contracts, forming a narrow, focused slurry jet that can penetrate the slurry pool and impinge on the bottom lining of the vessel. In some cases, such a high energy three-phase jet can cause extensive damage to the lining of the flash vessel. Often, normal shocks are formed inside the flash tube as well as oblique shocks and reflections downstream of the flash tube in the vessel volume itself due to over-expanded flows